The blood comprises a variety of different cells types. The most common cell type, the erythrocyte or red blood cell, which is responsible for carrying oxygen from the heart and lungs to the rest of the body, represents approximately 90% of the cells circulating in the blood and makes up approximately 45% of blood volume. Platelets, which are responsible for forming blood clots in response to injury, represent approximately 10% of the cells in the blood. Leukocytes, or white blood cells, generally comprise less than 0.5% of the cells in circulation, and subtypes of these cells (basophils, eosinophils, etc.) or other cells of interest, such as blood-borne bacteria or parasites, circulating stem cells, or mature leukocytes that are activated, transformed, cancerous, pre-cancerous, etc. are relatively rare, occurring at rates of fewer than one per ten-thousand cells.
Accurate quantification of these relatively rare cell populations is important in many areas of biomedicine. Cells in the blood are currently quantified either by extraction and analysis of small blood samples using, e.g., a hemocytometer, or using microscopy-based in vivo fluorescence flow cytometry. However, current approaches suffer from important limitations: first, their sensitivity is limited by the small number of cells that can be sampled using these methods; second, it is difficult to measure changes in cell populations over time; and third, extracted samples for quantitation can be cumbersome to handle, and can generate biohazardous waste.